I concur with everything you have said and have found exactly the same having done everything you have said. One more thing is that if you use FSLTL, turning down some of the settings in that can provide a significant boost in performance and smoothness and whilst this is an excellent addon, it can be a bit intensive at deault settings.
I also run with a LOD at 300 very smoothly having previously done all the things you suggest.
One other thing I do is limit FPS to 30 unless I am testing which reduces system load and increases smoothness considerably.
My recent upgrades to a 5800X3D, RTX3080 and 2TB NVME have of course helped too.